The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present disclosure broadly relates to compositions and methods for controlling lost circulation during the drilling of a wellbore.
During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through the wellbore to the surface. During this circulation, the drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
Fluid compositions used for these various purposes may be water- or oil-based and may comprise weighting agents, surfactants, proppants, or polymers. However, for a wellbore fluid to perform all of its functions and allow wellbore operations to continue, the fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the wellbore fluid may be lost to the formation. For example, wellbore fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
Lost circulation is a recurring drilling problem, characterized by loss of drilling mud into downhole formations. It can occur naturally in formations that are fractured, highly permeable, porous, cavernous, or vugular. These earth formations can include shale, sands, gravel, shell beds, reef deposits, limestone, dolomite, and chalk, among others. Other problems encountered while drilling and producing oil and gas include stuck pipe, hole collapse, loss of well control, and loss of or decreased production. An industry survey (James K. Dodson Co.) showed that, during the period 1993-2002, lost circulation problems related to wellbore instability accounted for 44% of non-productive time during the drilling process. With the development of extended reach drilling and the increased emphasis on deep water, lost circulation may account for an even larger share of non-productive time.
Lost circulation may also result from induced pressure during drilling. Specifically, induced mud losses may occur when the mud weight, required for well control and to maintain a stable wellbore, exceeds the fracture resistance of the formations. A particularly challenging situation arises in depleted reservoirs, in which the drop in pore pressure weakens hydrocarbon-bearing rocks, but neighboring or inter-bedded low permeability rocks, such as shales, maintain their pore pressure. This can make the drilling of certain depleted zones impossible because the mud weight required to support the shale exceeds the fracture pressure of the sands and silts.
Fluid losses are generally classified in four categories. Seepage losses are characterized by losses of from about 0.16 to about 1.6 m3/hr (about 1 to about 10 bbl/hr) of mud. They may be confused with cuttings removal at the surface. Seepage losses sometimes occur in the form of filtration to a highly permeable formation. A conventional LCM, particularly sized particles, is usually sufficient to cure this problem. If formation damage or stuck pipe is the primary concern, attempts are generally made to cure losses before proceeding with drilling. Losses greater than seepage losses, but less than about 32 m3/hr (about 200 bbl/hr), are defined as partial losses. In almost all circumstances when losses of this type are encountered, regaining full circulation is required. Sized solids alone may not cure the problem. When losses are between about 32-48 m3/hr (200-300 bbl/hr), they are called severe losses, and conventional LCM systems may not be sufficient. Severe losses particularly occur in the presence of wide fracture widths. As with partial losses, regaining full circulation is required. If conventional treatments are unsuccessful, spotting of LCM or viscous pills may cure the problem. The fourth category is total losses, when the fluid loss exceeds about 48 m3/hr (about 300 bbl/hr). Total losses may occur when fluids pumped past large caverns or vugs. In this case, the common solution is to employ cement plugs and/or polymer pills, to which LCM may be added for improved performance. An important factor, in practice, is the uncertainty of the distribution of zones of these types of losses, for example, a certain size fracture may result in severe loss or total loss depending on the number of such fractures downhole.
The use of fibers and solids to prevent lost circulation during drilling operations has been widely described. Such fibers include, for example, jute, flax, mohair, lechuguilla fibers, synthetic fibers, cotton, cotton linters, wool, wool shoddy, and sugar cane fibers. One known process for preventing or treating lost circulation involves the addition, at concentrations ranging between about 1.43 and about 17.1 kg/m3 of water-dispersible fibers having a length between about 10 and about 25 mm, for instance glass or polymer fibers, to a pumped aqueous base-fluid including solid particles having an equivalent diameter of less than about 300 microns. Another known process utilizes melt-processed inorganic fibers selected from basalt fibers, wollastonite fibers, and ceramic fibers. Such known methods and compositions, however, typically require large amounts of fibers.